This application relates generally to a method and apparatus for diffracting light, and more specifically to a diffraction grating useful in various applications, such as optical telecommunications, that require high diffraction efficiency in multiple polarization orientations.
The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today, DWDM systems using up to 80 channels are available from multiple manufacturers, with more promised in the future. Such systems are typically configured for operation in the telecommunications C band, which includes wavelengths between 1530 and 1565 nm.
Optical wavelength routing functions often use demultiplexing of a light stream into its many individual wavelengths, which are then optically directed along different paths. Subsequently, different wavelength signals may then be multiplexed into a common pathway. Within such routing devices, the optical signals are routed between the common and individual optical pathways by a combination of dispersion and focusing mechanisms. The focusing mechanism forms discrete images of the common pathway in each wavelength of the different optical signals and the dispersion mechanism relatively displaces the images along a focal line by amounts that vary with the signal wavelength.
Both phased arrays and reflective diffraction gratings may be used to perform the dispersing functions. While phased arrays are adequate when the number of channels carrying different wavelength signals is small, reflective diffraction gratings are generally preferable when large numbers of channels are used. However, reflective diffraction gratings tend to exhibit greater polarization sensitivity and since the polarization of optical signals often fluctuates in optical communication systems, this sensitivity may result in large variations in transmission efficiency. Loss of information is possible unless compensating amplification of the signals is used to maintain adequate signal-to-noise ratios. Although polarization sensitivity may generally be mitigated by increasing the grating pitch of the reflective grating, limitations on the desired wavelength dispersion for signals at optical telecommunication wavelengths preclude an increase in grating pitch sufficient to achieve high diffraction efficiency in all polarization directions.
It is therefore desirable to provide a diffraction grating that can achieve high diffraction efficiency without significant polarization sensitivity when used at C-band optical telecommunication wavelengths.
Thus, embodiments of the invention provide a reflective lamellar diffraction grating suitable for a variety of applications, including applications related to C-band telecommunication functions. In certain embodiments, the average efficiency of the diffraction grating in S- and P-polarization states exceeds 90% while simultaneously providing a PDL less than 0.2 dB over the entire wavelength range used for C-band telecommunication functions. The diffraction grating is thus suitable for incorporation into various telecommunication systems, including a wavelength router configured for routing signals having a plurality of spectral bands.
Accordingly, in a first set of embodiments, the lamellar diffraction grating comprises a substrate and an arrangement of generally rectangular protrusions spaced along a surface of the substrate at an average grating period a. The protrusions have an average height h and an average width w which are defined so that h/a greater than 0.5 and w/a less than 0.5. The diffraction grating thus has a profile in which the protrusions are generally both narrow and deep. In one embodiment, the protrusions have substantially equal heights and have substantially equal widths. The width of each protrusion may be defined by a FWHM measurement of a profile of such protrusion. For telecommunications applications, the grating period corresponds to a line density 1/a between 700 and 1100 protrusions/mm, and may correspond to a line density between 800 and 1000 protrusions/mm. Certain embodiments correspond to average heights and widths that provide particular efficiency and PDL behaviors: in one embodiment h/a is between 0.7 and 1.1, and w/a is between 0.15 and 0.3; in another embodiment h/a is between 0.75 and 1.0, and w/a is between 0.2 and 0.3; in a further embodiment h/a is between 0.84 and 0.96, and w/a is between 0.22 and 0.3.
As a result of the narrow and deep character of the protrusions, they may be fragile. Accordingly, further embodiments provide a method for fabricating such a grating and account for the fragility of the protrusions. A pattern for an anisotropic hard etch mask is formed over a surface of a substrate. The pattern has a period corresponding to the average grating period a of the diffraction grating to be produced. It also defines a width corresponding to the average protrusion width w of the diffraction grating. A plurality of gaps are etched into the substrate through the patterned anisotropic hard etch mask to an average depth that corresponds to the average protrusion height h of the diffraction grating. Such etching may be performed by using an anisotropic chemical etching technique. The pattern for the anisotropic hard etch mask may be formed by depositing the etch-mask material over the substrate and forming a layer of photoresist over the etch-mask material. The pattern is exposed onto the layer of photoresist and the anisotropic hard etch mask is etched through the pattern in the layer of photoresist, such as with an isotropic reactive ion etching technique. The layer of photoresist is subsequently removed, such as by application of an organic solvent.
In specific embodiments, such reflective lamellar diffraction gratings may be used in wavelength routers. One such embodiment includes a free-space optical train disposed between an input port and a plurality of output ports and provides optical paths for routing light having a plurality of spectral bands. The optical train includes a reflective lamellar diffraction grating with the characteristics described above and is disposed to intercept light traveling from the input port.